1. Field of the Invention
The present invention relates generally to the field of diemaking and particularly dies used to cut and score cardboard, paperboard, and other materials to produce folding carton blanks.
2. Description of the Relevant Art
Steel rule dies have been used for many years to cut and score cardboard, paperboard and certain plastic materials used to produce folding carton blanks. It is conventional to manufacture such dies by cutting slots in a die base and inserting steel rules in the slots which extend above the surface of the die base and thereby perform the cutting and scoring operations on the carton blanks. The slots are generated by jig saws, and more recently, by the use of laser beams.
For many years, the practice used to retain the steel rules within the slots has been to control the width of the slots so that the steel rules are held in place by frictional fit contact with the slot sidewalls. Once in place, a side force is exerted by the slot walls against the rules which serves to positively hold them in position. This arrangement has not been entirely satisfactory however because the die base, which is typically wood, is inherently susceptible to dimensional changes, i.e. expansion, contraction and warping, due to changes in the moisture content of the wood, temperature, and other environmental conditions. These dimensional changes can cause the rules to become loosened in their slots, necessitating unscheduled repairs and machine downtime.
Because of problems such as these, base materials other than wood have been tried, as well as other types of die constructions. U.S. Pat. No. 4,052,886 for example describes a steel rule die having a base which is cavernous to receive a filler material which is poured therein and cured to a semi-rigid state to anchor the rules. This arrangement is disadvantaged in that it requires repouring of the die to resecure any loosened rules and does not provide for any side holding force on the rules to retain them in their slots.
U.S. Pat. No. 3,863,550 discloses a steel rule die having a sandwich type base including two plates between which is poured a thermoplastic or thermosetting resin material capable of being cured to a semi-rigid structure. In addition to being expensive to manufacture, this arrangement does not provide any side force against the rules to retain them inside the rule slots.
U.S. Pat. No. 5,029,505 discloses a holder for holding rules in steel rule dies. The holder comprises a ball/spring assembly which exerts a side force against the rule which serves to retain the rule in its associated slot. Although this design provides a side force to positively retain the rule in its slot, there are other disadvantages which reduce the desirability of this arrangement. For example, the holders have dimensional limitations owing to the ball/spring/housing assembly which make them difficult or impossible to install in conjunction with rule segments of relatively short length. Further, corrosion can build up on the spring and ball assembly and reduce its effectiveness. During cutting or creasing operations it is not unusual to create samll particles of paper dust which can become lodged in the spring so as to jam the spring mechanism. In addition, the holders can themselves become loose and possibly dislodge from their holding channels as a result of the expansion or contraction of the base material defining the holding channels.